The present invention relates generally to the transfer of microwave energy, and more particularly to the coupling of microwave energy through dielectric filled longitudinal slots in the common wall of concentric circular and coaxial dielectric-lined waveguides. The invention has particular application to microwaves of very high power, as might be used with megawatt level gyrotrons.
Waveguides are a form of transmission line used to transmit electromagnetic energy efficiently from one point to another. Waveguides may be rectangular, circular or coaxial. The type of waveguide that is best suited for a particular application depends upon how the microwave energy is being used. If it is being coupled from a source of microwave energy, e.g., a gyrotron, one type of waveguide may be more efficient (i.e., couple the energy with less loss) than another type of waveguide. If it is being transmitted over relatively long distances, another type of waveguide may be more efficient. If it is being delivered to a load, still another type of waveguide may be more efficient. Hence, there is a recurring need for waveguide couplers that efficiently couple microwave energy from one type of waveguide, e.g., coaxial, to another type of waveguide, e.g., circular.
Waveguide modes are denominated to identify the distribution of the electric and magnetic fields within the waveguide. As indicated in the art, e.g., Electronics Designers' Handbook, 24 Edition (McGraw-Hill 1977) at page 8-36; or Marcuvitz, N., Waveguide Handbook, McGraw-Hill, pp. 72-80 (1951), specific modes are indicated by symbols such as TE.sub.mn and TM.sub.mn. TM indicates that the magnetic field is everywhere transverse to the axis of the transmission line, i.e., the longitudinal axis of the waveguide. TE indicates that the electric field is everywhere transverse to the axis of the waveguide. For rectangular waveguides, the subscripts m and n denote the number of half period variations of the fields occurring within the waveguide in the two transverse dimensions. For circular waveguides, the subscript m denotes the number of full-period variations of the transverse component of the field in the angular direction, and is frequently referred to as the angular mode number, while the subscript n denotes the number of half-period variations of the transverse component of the field in the radial direction. A circular waveguide mode having no angular dependence may thus be either a TE.sub.0n or a TM.sub.0n mode, where n is any integer.
A common waveguide mode useful with the new generation of millimeter wavelength gyrotrons, having, e.g., output frequencies greater than 100 GHz and output power greater than 500 kW, is the hybrid HE.sub.11 mode. The HE.sub.11 mode, for purposes of this application, may be regarded as a superposition of a conventional TE mode and a conventional TM mode that exists only in certain types of waveguides, e.g., corrugated waveguides. See, e.g., C. Dragone, "High-Frequency Behavior of Waveguides with Finite Surface Impedances", Bell System Tech J. 60: 89-116 (1981).
As shown hereinafter, another type of waveguide that supports the hybrid modes is a dielectric-lined circular waveguide.
Unlike other waveguide modes, microwave energy propagating in the HE.sub.11 mode couples well to free space waves after the waveguide terminates. Hence, the HE.sub.11 mode is highly desirable for applications, such as communications or plasma heating, that require the microwave energy to be used outside of the waveguide and focused or otherwise directed to a desired target or zone. Unfortunately, the output power available from most high power gyrotrons, including the quasi-optic gyrotrons, is not available in the HE.sub.11 mode. Hence, there is a need for, microwave waveguide mode converters that efficiently convert microwave energy from whatever mode is available at a source of the microwave energy to a mode more useful for a desired application.
Frequently, the coupling of microwave energy from one location to another, e.g., from a first waveguide to a second waveguide, must be achieved while maintaining an appropriate seal between the coupled locations. Waveguide windows are used to permit power to pass from one waveguide to a second waveguide, while maintaining a physical barrier between the two waveguides. The seal or physical barrier is required because, e.g., the waveguides may contain different gases or have different pressure levels, and one or both waveguides may be evacuated. Moreover, in high power microwave vacuum devices, such as gyrotrons and the like, power is generally transferred between an evacuated chamber or waveguide in the device and a waveguide having a gaseous environment. One or more waveguide windows may thus be used to provide a hermetic seal between the two media.
In addition to requiring a window at a gyrotron output, a window may also be needed near a destination site of the microwave power, i.e., at the load, to provide a suitable barrier or shield from undesirable elements at the destination site. For example, where the destination site includes a plasma device, a significant amount of tritium may be present. Thus, even if the transmission line from the gyrotron is evacuated, a window may be needed to serve as a tritium barrier.
An output window used within a gyrotron must also frequently be coupled to a suitable collector that absorbs the electron beam utilized in such devices. Unfortunately, existing collectors used within gyrotrons have low microwave loss and low mode conversion, which compromises their design as electron beam dumps.
From the above, it is evident that there is a need in the art for a microwave coupler, window and converter device that may be used, e.g., with a high power gyrotron to efficiently couple the output power of the gyrotron through a window barrier to an output waveguide, while at the same time converting the waveguide mode of the microwave energy in the output waveguide to a desired waveguide mode. The present invention advantageously addresses these and other needs.